Horse Owners: Vaccinate against Mosquito-Borne Diseases

West Nile virus (WNV) and Eastern equine encephalomyelitis (EEE) are endemic across North Carolina and can cause illness or death in equids, but can be prevented with a series of two vaccines.

Photo: Erica Larson, News Editor

It’s the beginning of mosquito season in North Carolina, which means it’s also time for horse owners to talk to their veterinarians about vaccinating animals against mosquito-borne diseases.

West Nile virus (WNV) and Eastern equine encephalomyelitis (EEE) are endemic across North Carolina and can cause illness or death in equids, but can be prevented with a series of two vaccines. Last year, there were two reported cases of WNV and 13 cases of EEE in North Carolina, but area veterinarians expect that the actual number is higher.

“Now is the time to vaccinate against West Nile virus and EEE,” said State Veterinarian David Marshall, DVM. “Mosquito breeding peaks in August, so starting the vaccination protocol now gives it time to take effect.”

The EEE and WNV vaccinations initially require two shots, three to four weeks apart, for horses, mules, and donkeys that have no prior vaccination history. Neither vaccination fully protects the animal until several weeks after the second shot, so it is best to vaccinate as early in the mosquito season as possible. Previously vaccinated horses should be boostered at least annually. Marshall encourages horse owners to talk to their veterinarians about maintaining the vaccination year-round in North Carolina, since the mosquito season is long.

“In addition to getting animals vaccinated, everyone needs to be extra vigilant now to reduce the breeding grounds for mosquitoes,” Marshall said. “Take the time now to rid your yard and pasture of any standing water to reduce the risk.”

A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. Clinical signs of EEE include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs despite intensive care; fatality rates reach 75-80% among horses. Horses that survive might have long-lasting impairments and neurologic problems.

Like EEE, WNV is a viral disease transmitted to horses by infected mosquitoes. Clinical signs of WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculations (twitching); hyperesthesia (hypersensitivity to touch and sound); changes in mentation (mentality), when horses look like they are daydreaming or "just not with it"; occasional drowsiness; propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia (incoordination). Equine mortality rate can be as high as 30-40%.

People, horses, and birds can become infected from a mosquito carrying the diseases, but there is no evidence that horses can transmit the viruses to other horses, birds, or people through direct contact.

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